Various household electrical appliances including a television and a refrigerator are operated on external commercial alternating current power. Also, electronic devices including a laptop computer, a mobile phone, and a tablet PC can be operated on external commercial alternating current power or can charge batteries embedded in the electronic devices by using external commercial alternating current power. These household electrical appliances or electronic devices (hereinafter, generically referred to as “electronic devices”) are embedded with power devices (inverters) for AC/DC (alternated current/direct current) converting an external commercial alternating current voltage. Alternatively, the inverters may be embedded in power adaptors (AC adaptors) outside the electronic devices.
FIG. 1 is a block diagram of an AC/DC converter 400r reviewed by the inventor of the present disclosure. Major components of the AC/DC converter 400r include a rectification circuit 402, a smoothing capacitor 404, and a DC/DC converter (switching converter) 100r. 
The rectification circuit 402 is a diode bridge circuit that performs full-wave rectification of a commercial alternating current voltage VAC. An output voltage of the rectification circuit 402 is smoothed by the smoothing capacitor 404, and is converted to a DC voltage VDC.
The DC voltage VDC is supplied to an input line 104 of the insulated DC/DC converter 100r at the next stage. The DC/DC converter 100r generates an output voltage VOUT that is stabilized to a target level by dropping the direct current VDC and supplies the output voltage VOUT to a load (not illustrated) connected to an output line 106.
The DC/DC converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a switching transistor M1, a current sensing resistor RCS, a transformer T1, a rectifier diode D1, an output capacitor C1, and a feedback circuit 108. The feedback circuit 108 generates a feedback voltage VFB based on the output voltage VOUT and supplies the feedback voltage VFB to a feedback terminal (FB terminal) of the control circuit 200r. 
The switching transistor M1 and the current sensing resistor RCS are connected to a primary coil LP of the transformer T1. The rectifier diode D1 and the output capacitor C1 are connected to a secondary coil Ls of the transformer T1.
An output terminal OUT of the control circuit 200r is connected to a gate of the switching transistor M1. The control circuit 200r generates a pulse signal SOUT having a duty ratio adjustable to make the output voltage VOUT approach a predetermined target level, and switches the switching transistor M1.
The control circuit 200r is configured to be capable of detecting a current flowing in the primary coil LP and the switching transistor M1 (hereinafter, referred to as a “coil current IP”) during the ON period of the switching transistor M1. In detail, a current sensing terminal (CS terminal) of the control circuit 200r is connected to the current sensing resistor RCS, and a sensed voltage VCS in proportion to the coil current IP is input to the CS terminal. A current comparing circuit 300r compares the sensed voltage VCS with a predetermined threshold voltage VTH, thereby comparing the coil current IP with a threshold current ITH (=VTH/RCS). Described above is the configuration of the AC/DC converter 400r. 
FIG. 2 is an operation waveform diagram of the DC/DC converter 100r. When the pulse signal SOUT is at a high level, the switching transistor M1 is turned on. When the switching transistor M1 is turned on, the coil current IP increases as time lapses, and thus the sensed voltage VCS increases. When the pulse signal SOUT is at a low level, the switching transistor M1 is turned off. During the OFF period of the switching transistor M1, the current IS flows in the secondary coil LS and is supplied to the output capacitor C1. The output voltage VOUT is stabilized to a desired level by repeatedly switching the switching transistor M1.
During the ON period of the switching transistor M1, the coil current IP flows in the primary coil LP and the DC voltage VDC is applied to between both ends of the primary coil LP. Accordingly, Equations (1) and (2) are established.VDC==LP·dIP/dt  (1)VCS=RCS×IP  (2)Equation (3) can be obtained by modifying the above Equations (1) and (2).VCS=RCS/LP×∫VDCdt=(RCS/LP×VDC)×t  (3)In Equation 3, (RCS/LP×VDC) denotes the slope [V/s] of the sensed voltage VCS, which will be hereinafter referred to as “α”. In other words, the slope α of the sensed voltage VCS during the ON period of the switching transistor M1 depends on the DC voltage VDC and the inductance of the primary coil LP.
FIG. 3A is an operation waveform diagram of the current comparing circuit 300r, and FIG. 3B is a diagram illustrating an effective threshold voltage. A comparator of the current comparing circuit 300r has a response delay τD, and an output signal SCMP of the comparator transits after a lapse of the response delay m from the time when it becomes VCS=VTH. The sensed voltage VCS when the output signal SCMP of the comparator is changed is referred to as an effective threshold voltage VTH_EFF. As illustrated in FIG. 3A, as the slope α of the sensed voltage VCS increases, the effective threshold voltage VTH_EFF becomes higher than the ideal threshold voltage VTH. The effective threshold voltage VTH_EFF_is given as Equation (4).VTH_EFF=VTH+α×τD  (4)
Therefore, in a case where the output SCMP of the comparator is used for overcurrent protection and the like, the slope α is changed due to fluctuation of the DC voltage VDC or fluctuation (deviation) of a coil LP. Accordingly, the effective threshold voltage VTH_EFF, and further, a threshold current ITH is changed or fluctuates.
In the related art, there is known a technique for suppressing fluctuation of a threshold current ITH according to fluctuation of an input voltage VIN. In detail, a threshold voltage VTH(t) that increases as time lapses from turn-on of a switching transistor M1 is generated, and is compared with a sensed voltage VCS.
FIG. 4 is a waveform diagram illustrating a current detection in the related art. The slope of the sensed voltage VCS depends on the input voltage VIN, and FIG. 4 illustrates (i) a case where the input voltage VIN is high and (ii) a case where the input voltage VIN is low. At a time point t=0, the switching transistor M1 is turned on. As the switching transistor M1 is turned on at the time point t=0, a threshold voltage VTH(t) gradually increases. Therefore, the threshold voltage VTH(t) gets higher as the time lapses from the transition to the ON period TON.
The effective threshold voltage VTH_EFF is the sensed voltage VCS obtained at a time point after a lapse of the response delay m of the comparator from intersection of the sensed voltage VCS with the threshold voltage VTH(t).
A voltage width (overshoot amount) of the sensed voltage VCS exceeding the threshold voltage VTH(t) during the response delay τD of the comparator increases as the slope of the sensed voltage VCS increases. However, the level of the threshold voltage VTH(t) also increases as the time lapses during the ON period TON. Therefore, an overshoot amount ΔV can be cancelled, thereby suppressing fluctuation or deviation of the effective threshold voltage VTH_EFF.
After reviewing the current detection method in the related art using the time-varying threshold voltage VTH, the inventor of the present disclosure recognized the below technical problems. As illustrated in FIG. 4 (in particular, VCS(ii)), when the input voltage VTH(t) is lowered, the slope of the sensed voltage VCS approaches the slope of the threshold voltage VTH(t). If the slopes of the two voltages VCS and VTH(t) approach to each other, due to noise or an offset of the comparator, it becomes difficult to precisely compare the voltages VCs and VTH(t) having voltage levels close to each other. In detail, there may be a problem in that the output of the comparator is changed while the two voltages VCS and VTH(t) do not intersect with each other or in that the output of the comparator is not changed while the two voltages VCs and VTH(t) intersect with each other. In some cases, chattering may occur.